Jump to a function's source from the traced-buffer view

In the traced-buffer view (`sayid-show-traced`), pressing RET (`sayid-traced-buf-enter`) on a *namespace* node shows that namespace's trace view. On a *function* node it currently does nothing - there's a long-standing placeholder branch (commented `goto func`) that was never implemented.

It would be a nice UX win to have RET on a function node jump to that function's definition. That needs the middleware to resolve the function's source location (file + line), which Sayid already tracks for traced fns, and the client to open it.

The dead placeholder branch is being removed as part of the Elisp cleanup; this issue tracks building the real thing.

Start at the sayid-traced-buf-enter handler used by sayid-show-traced and inspect the middleware path for traced functions. Follow how Sayid already tracks a function's source file and line, then connect that location to the client behavior. Done means RET on a function node opens its definition, while namespace-node navigation remains unchanged.

Written by the indexing model from the issue text.